A type of stepless transmission, so-called a Trotrak Continuously Variable Transmission, provided with a toroidal groove having a circular cross-section is formed around the common rotation axis of apposite input and output disks, and a plurality of disks referred to as power rollers are held In this groove. The input disk is fixed on a rotation shaft while the output disk is supported such that it is free to rotate around the shaft.
The power rollers come into contact with these input and output disks across the circular cross-section of the toroidal groove, therefore, when the input disks rotate, the power rollers rotate due to friction, and this rotation is transmitted to the output disks by the friction. The rotation speed of the power rollers is faster the greater the distance of the contact point of the input disk from the rotation axis. Further, the rotation speed of the output disk is faster the nearer the contact point of the output disk to the rotation axis. These contact points may be freely changed by modifying the inclination of the power rollers.
The rotation axes of the power rollers are supported at one end of an extensible arm, this extensible arm being supported such that it is free to turn about its own center axis. When this arm is extended, the slant angle of the power rollers varies depending on the circular cross-sectional shape of the toroidal groove.
The transmission ratio may therefore be freely adjusted by controlling an actuator which drives the extensible arm.
In transmissions where a mechanism is provided before the input disks for changing over between forward and backward drive by reversing the rotation direction, the rotation direction of the input disks will be different according to whether the vehicle is driven forward or backward, and the inclination of the power rollers must be adjusted accordingly.
In Tokkai Hei 2-163562 published by the Japanese Patent Office, for example, a mechanism is disclosed wherein the extensible arm is driven by an oil pressure actuator, the oil pressure supplied to the actuator being supplied by different controllers according to whether the vehicle is driven forward or backward.
In this mechanism two oil pressure controllers are selectively connected to the actuator via a spool valve which is displaced in an axial direction between a forward drive position mid a backward drive position. The spool valve is driven via a transfer shaft whose motion is correlated with that of the rotation shaft of the input disk, a collar fitted to the transfer shaft a one-way clutch engaging the transfer shaft with the collar, am arm fixed to the collar which drives the spool valve, and a frictional torque transfer mechanism which transfers a rotational torque within predetermined limits between the collar and the arm.
The one-way clutch rotates freely when the transfer shaft rotates in the forward drive direction, and is locked when the transfer shaft rotates in the backward drive direction. The torque of the transfer shaft is therefore transferred to the collar only when the vehicle is driven backward. The torque transfer mechanism is provided with a sliding surface which slides tinder the action of a torque above a predetermined limit. The spool valve is maintained in the forward drive position by a spring, and when backward driving is desired, the arm which is driven by the collar moves the spool valve to the backward drive position against the force of the spring.
The arm which has driven the spool to the backward drive position does not move any further due to the sliding of the sliding surface of the transfer mechanism, and the spool is therefore held in the backward drive position.
When the transfer shaft changes the rotation direction to forward drive, however, the one-way clutch rotates freely, the arm is released from the rotational torque, and spool drive force is lost. The spool which was pushed by the spring therefore moves to the forward drive position.
In this change-over mechanism, the spool is driven at the end of the arm, and the arm drive torque is supplied from the transfer shaft. The torque transfer mechanism is situated between the base of the arm and the collar, and as the arm is nearer to the transfer shaft than the action point of the force acting oil the spool, the sliding friction of the sliding surface of the torque transfer mechanism must be set at a considerably higher level in order to transfer the torque required by the arm. If this sliding friction is set too high, however, the sliding surface is easily worn down by sliding when the vehicle is driven backward, and durability is poor.
Moreover, this mechanism involves complex structures such as the one-way clutch, and there are too many parts between the transfer shaft and the spool.
Still further, when fitting the spool valve to the transfer mechanism, the arm has to be drawn back so that it is not damaged by the spool, and this rendered the assembly procedure somewhat troublesome.